Hi,
I noticed that when I installed grub-btrfs via pacman there was no service for openrc, but strangely there was one installed for systemd.
According to the package contents it installs "/usr/lib/systemd/system/grub-btrfsd.service" but nothing for /etc/init.d.
When searching in the grub-btrfs github repository there appears to be a service specifically for openrc (Under the filename grub-btrfsd.initd) which i used as a workaround for now.
https://packages.artixlinux.org/packages/galaxy/any/grub-btrfs/